1. Field
The following description relates to a multicast/broadcast service (MBS), and more particularly, to an MBS data transmission synchronization apparatus and method that perform transmission synchronization of MBS data transmitted from a wireless network to a mobile terminal.
2. Description of the Related Art
In the WiBro/Wi-Fi network, mobile terminals included in the same multicast group receive the same MBS contents. MBS contents are transmitted as an MBS flow that is represented as a multicast connection identifier (MCID) and security aggregation (SA) in a base station. An MBS zone indicates a zone in which one MBS flow is valid. MBS contents that are transmitted to mobile terminals in the same MBS zone are required to be synchronized for obtaining a macro diversity gain.
Mobile terminals receive MBS contents of an MBS content source through an access control router (ACR) and a radio access station (RAS), namely, a base station. Here, the MBS contents and the ACR are connected by cable, and the ACR and the RAS are connected wirelessly. That is, the RAS transmits MBS contents received through the ACR to mobile terminals located in a cell over a wireless network.
However, even though the MBS content source transmits MBS contents to different RASs in the same MBS zone at the same time, the different RASs may receive the MBS contents at different times. For example, times at which the RASs receive the MBS contents may differ according to paths from the MBS content source to the different RASs in the same MBS zone.
Since different RASs receive MBS contents at different times, mobile terminals located in a cell for each of the RASs may receive different content packets. For example, if a time at which a first RAS in the same MBS zone transmits MBS contents is earlier by two frames than a time at which a second RAS in the same MBS zone transmits MBS contents, mobile terminals located in the first RAS receive the third packet of the MBS contents when mobile terminals located in the second RAS receive the first packet of the MBS contents.
MBS content transmission times of RASs differ, and thus, when a mobile terminal performs handover to a different RAS in the same MBS zone while receiving MBS contents, it is unable to seamlessly receive MBS contents. That is, the mobile terminal receives MBS contents degraded by redundancy or loss.
Therefore, to overcome such limitations, Korean Patent Publication No. 10-2008-0073553 discloses an apparatus for synchronizing transmission of multicast/broadcast service data and a method using the same that synchronize the transmission time of MBS contents to an RAS and retransmit a lost MBS content packet.
However, in the above-described conventional technology, the synchronization apparatus operates on the assumption that contents consisting of transmission synchronization information and packet data are all transmitted to a plurality of base stations without error. That is, when at least one of a plurality of base stations in the same MBS zone cannot receive transmission synchronization information or receive all or a portion of packet data, information and packet data that have been already lost before determining a sync transmission time are transmitted to a mobile terminal. Therefore, when a mobile terminal receives MBS contents and simultaneously performs handover to a different RAS in the same MBS zone, seamless reception of MBS contents cannot be ensured, nor can synchronization of MBS contents transmitted to mobile terminals in the same MBS zone for obtaining a macro diversity gain. Thus, since a base station that has not received transmission synchronization information cannot determine a transmission time, a mobile terminal receives MBS contents degraded by redundancy or loss. Accordingly, a method is required in which a plurality of RASs are guaranteed to receive transmission synchronization information and reliable packet data contents prior to determining a sync transmission time.